1. Description of Prior Art
A wireless sensor network system has been used as a system which includes a plurality of sensor terminals and collects data of equipment monitoring and data of power, gas and water usage in a factory or a plant through wireless communication. In such a wireless sensor network system, in many cases, a sensor terminal may access a server or the like via a wired network in order to manage data collected from sensor terminals. A sensor terminal, which has the function of access the wired network and arranges and transmits data from a sensor terminal within a network to a server, is referred to as a gateway terminal (GW terminal).
A specified low power wireless device is often used as the wireless terminal (sensor terminal) used in a wireless sensor network system. Although the maximum wireless transmission power of the specified power wireless device is limited to 10 mW, increase of the upper limit is being considered.
In addition, in the wireless sensor network system, one network may include several thousands to tens of thousands sensor terminals, and accordingly, in order to prevent inter-sensor terminal interference and reduce power consumption, it is desirable to restrict wireless transmission power and wireless transmission frequency.
Accordingly, for the purpose of securing reliability of wireless communication quality, each sensor terminal performs communication at a wireless transmission power set by considering to allow reliable communication even in case of deteriorated communication quality of a propagation path, without performing a control dynamically and finely adjust transmission power as in a mobile phone or a wireless data module. It is common that the wireless transmission power is set when the sensor terminal is installed.
In the wireless sensor network system, sensor terminals participate in the network via adjacent terminals in an ad-hoc way, to sequentially form a network topology (communication route).
Then, each node (sensor terminal) conducts communication by sequentially transmitting (hopping) a traffic to the next node. This multi-hop communication can cover a wide area.
However, transmission delay may be increased with an increase in the number of hops from the GW terminal, and, depending on arrangement of sensor terminals, a traffic load may be concentrated on a specific sensor terminal and data transfer delay through the specific sensor terminal may be increased.
2. Related Art
One of techniques related to the wireless sensor network system is disclosed in Japanese Patent Application Publication No. 2009-296262 (Title: Radio Communication Device) (Hitachi Kokusai Electric Inc., Patent Document 1). As disclosed in Patent Document 1, a terminal includes a narrow band radio device having a wide coverage and a wide band radio device having a narrow coverage. In conduction of communication, destination information is transmitted from the narrow band radio device and route information is received in the narrow band radio device from a destination node within a certain period of time. If the route information matches with stored information, the terminal performs communication with the reception destination node based on the route information by using the wide band device. In this way, the terminal achieves efficient utilization of radio resources.
However, in the conventional wireless sensor network system, depending on arrangement of sensor terminals, transmission delay may be increased with an increase in the number of hops and the traffic load may be concentrated on a specific sensor terminal.